1. Field of the Invention
This invention relates to a television set being capable of receiving and reproducing stereo sound.
More particularly, the invention relates to a television set comprising loudspeakers for reproducing a left channel sound signal and a right channel sound signal.
2. The Prior Art
Stereophonic listening to more than one sound reproducing source has been studied for many years (see: Jens Blauert: Raumliches Horen, Hirzel Verlag, Stuttgart, 1974, p. 161). A very important result of these researches was the finding that two sound sources are recognized as one sound source only, if the two sound signals were similar in phase and amplitude. This effect is called "Sum localisation". Another important finding was the so-called "Law of the first wave front" which means that only one out of two sound signals is recognized, if a time difference of about 1 Millisecond lies between two sound signals of similar levels or amplitudes. Differences in the transit time of more than one second will cause an echo-effect, if the amplitudes of two sound sources are similar. A variation of the level (H. D. Harwood "Stereophonic Image Sharpness", Wireless World, 1968, p. 207-210) as well as of the time difference of two sound signals has a mutual influence on the sound localisation or image position. In view of these experiments the best position for listening to two-channel stereo sound reproductions is between the two loudspeakers being capable of reproducing the left and right channel information, respectively. More specifically, the two loudspeakers and the listener should define the corners of an equilateral triangle (see: W. Baier: Rundum-Klang, radio-tv-electronic 1977, No. 12, p. 30,32, FIG. 1). If the distance from the listener to one of the two loudspeakers differs from the distance between the listener and the other loudspeaker, the stereo effect disappears in dependence upon the difference of the two distances. Thus, only a very small area in front of the two loudspeakers is suitable for a good stereo effect. This area is the smaller the smaller the so-called stereo basis is, i.e. the distance between the two loudspeakers.
In a television set comprising two loudspeakers, one being located at the left side of the television tube and the other being located at the right side of said tube, the stereo basis is very small, since television sets have a tube diameter of 30 to 70 cm only. Therefore, it is desirous to broaden the stereo area of a television set. Many proposals have been made in the art of radio stereophony to enlarge the stereo basis. One of those proposals concerns an electrostatic loudspeaker in a rectangular room, the diaphragm of this loudspeaker covering one wall of the room (P. J. Walker, Wide Range Electrostatic Loudspeakers, part 3--Complete Systems, Wireless World, August 1955, FIG. 6). Sound intensity throughout the room is independent of position. The apparent sound source is always in a direction perpendicular to the diaphragm and, of course, moves a the listener moves. The same loudspeaker may be used for stereophony. With transmission line matching and feeding the signal at one end the wavefront will be tilted, due to the time delay. Separate signals may be fed from either end to produce two tilted wavefronts, one for each signal. Since each apparent origin is perpendicular to its wavefront, the aspect angle from the listener is a constant and entirely independent of the listener's position over a large triangular area. The relativ intensity of the two signals is also constant. A similar principle using a plurality of electrodynamic loudspeakers instead of an electrostatic loudspeaker has recently been proposed (see: Verbesserung des Raumeindrucks durch Holophonie, Funkschau 1980, No. 5, p. 74-76). In this latter system, however, it is also made use of the fact that only frequencies of more than 300 Hz are useful for stereo location (see: K.de Boer, Plastische Klangwiedergabe, Philips Technische Rundschau, 1940, No. 5, p. 108-115). Consequently, only the frequencies above 300 Hz are led to a plurality of tweeters and midrange loudspeakers, whereas the lower frequencies are led to common woofers.